An optoelectronic assembly may include for example one, two or more light emitting diode elements. The light emitting diode elements can be for example light emitting diodes (LEDs) and/or organic light emitting diodes (OLEDs) or parts or segments of light emitting diodes (LEDs) and/or organic light emitting diodes (OLEDs).
Despite elaborate quality control procedures for light emitting diode elements, the situation in which the light emitting diode elements fail spontaneously in use cannot be completely ruled out. In the case of an OLED, for example, a typical fault pattern for a spontaneous failure is a short circuit (referred to as: short) between the electrodes of the corresponding light emitting diode element. Such a short circuit is generally over a small area. A large part of the total current is thus concentrated at the short-circuit point having a small area. Consequently, the current density is significantly boosted at the short-circuit point, as a result of which said short-circuit point can heat up greatly depending on its areal extent. This can lead to the melting of the electrodes, to dark spots in the luminous image of the OLED, to a completely dark OLED and/or to a location becoming hot on the OLED.
In order to prevent a potential hazard as a result of this overheating (combustion hazard, fire, rupture, etc.), such a short circuit should be identified by driver electronics of the optoelectronic assembly and a suitable protective reaction should be initiated (switching off of the OLED or of the optoelectronic assembly, bypassing of the supply current around the short-circuited OLED, outputting of a warning signal, etc.). In the automotive sector, for example, it is demanded that defective OLEDs or LEDs, for example in rear lights, be electronically identified and at least reported to the on-board system.
A customary interconnection of light emitting diode elements, for example OLEDs, of an optoelectronic assembly in use is, for technical reasons and for cost reasons, the series connection of the light emitting diode elements. By way of example, a plurality of light emitting diode elements in a light emitting diode can be connected in series and/or a plurality of light emitting diodes can be connected in series. In many applications, for example in the automotive sector or in the field of general lighting, a plurality of light emitting diode elements are thus electrically connected in series. If individual defective light emitting diode elements in a series connection are intended to be identified using simple methods, this constitutes a particular challenge.
US 2011 204 792 A1, WO 2010 060 458 A1 and WO 2012 004 720 A2 disclose methods for determining short circuits of individual OLEDs in which an overvoltage or undervoltage at the corresponding OLED is used as a criterion for a defect. As a reaction to the identification of the short circuit, the methods implement bypassing of the drive current and/or fault signal generation.
In the field of general lighting it is typically the case that flexible control devices have a variable output range. As a result, a variable number of light emitting diode elements can be connected to the control devices. The number actually connected is not known during the programming and/or development of the control devices. By way of example, between two and seven OLEDs can be connected to a typical driver circuit from the field of general lighting. The number is variable within the predefined scope, that is to say that fixed undervoltage identification thresholds cannot be defined in the case of the driver circuit. One input possibility at the driver circuit for inputting the number of connected light emitting diode elements is complex and expensive.
FIG. 1 shows a conventional optoelectronic assembly 10 including a first light emitting diode element 12, a second light emitting diode element 14, a third light emitting diode element 16 and a fourth light emitting diode element 18. The light emitting diode elements 12, 14, 16, 18 are arranged in a component string 22 of the optoelectronic assembly 10. The second light emitting diode element 14 has a short circuit, which is depicted as short-circuit resistance 24 in FIG. 1. The short-circuit resistance 24 is electrically connected in parallel with the second light emitting diode element 14 and behaves electrically similarly to an ohmic resistance, wherein the value of the resistance can vary depending on the type of short circuit.
With a measurement of the forward voltage in accordance with the conventional methods for determining the short circuit in the case of the optoelectronic assemblies 10 illustrated in FIGS. 1 and 2, the following problems arise if individual measurement is not carried out at each light emitting diode element 12, 14, 16, 18: The resistance value (R_Short) of the short-circuit resistance 24, for example in the case of an OLED, is in a wide range, for example of between 10 ohms and a number of kohms. With one input of the component string 22 and one output of the component string 22, only a total voltage (Utot) across all the light emitting diode elements 12, 14, 16, 18 can be detected during nominal operation. Given identical light emitting diode elements 12, 14, 16, 18, the total voltage thus corresponds to four times corresponding individual voltages (Uf) of the light emitting diode elements 12, 14, 16, 18 and, without a short circuit, results asUtot=4×Uf. 
If the short circuit is present in the case of one of the light emitting diode elements 12, 14, 16, 18, then the following results:Utot=3×Uf+R_Short×I. 
Given an individual voltage of Uf=6V, a nominal operating current (I) of 300 mA and a short circuit having a resistance value of 10 ohms, the total voltage results asUtot=3×6 V+10 ohms×0.3 A=21 V.
If the identification threshold (U_T) for the short circuit in the case of one of the light emitting diode elements 12, 14, 16, 18 is set to three and half times the individual voltage, then the identification threshold results asU_T=3.5×6 V=21 V.
Consequently, the total voltage in this example is exactly at the identification threshold, which does not yield sufficient identification certainty in the case of variations of the corresponding measurement values that occur in reality.
If the short circuit only has a resistance value of 50 ohms, for example, then the total voltage results asUtot=3×6 V+4.8 V=22.8 V,for which reason the short circuit with the above identification threshold U_T=21 V is not identified as such. This can stem from the fact that a corresponding short circuit can have a higher resistance than the organic system of the short-circuited OLED. The individual voltage of the corresponding OLED is thus principally determined by the organic system and not by the short circuit. Nevertheless, the current density is increased at the short-circuit point, which leads to the temperature increase, for which reason there should be a reaction to the short circuit.
In the case of flexible control devices for connecting different numbers of light emitting diode elements 12, 14, 16, 18, the conventionally identifiable reduction of the total voltage by a short circuit goes down as a percentage, in particular in the case of long string lengths, or is partly canceled out by the voltage drop at the short circuit and is thus tolerance-susceptible. A short-circuit signature present in the case of the total voltage is identifiable with difficulty or not at all, since, in the case of undefined string lengths, a dedicated fault threshold would have to be defined for each string length.
The problems thus arise that, in the case of a short circuit, the individual voltage across the short-circuited light emitting diode element 12, 14, 16, 18, owing to the voltage drop at the short circuit during nominal operation, does not necessarily drop significantly compared with a light emitting diode element 12, 14, 16, 18 without a short circuit, and that, in the case of an unknown number of light emitting diode elements 12, 14, 16, 18, in principle it cannot be identified whether the total voltage is normal or lower than normal owing to a short circuit.
Therefore, it is known to provide just one light emitting diode element per driver circuit, that is to say no series connection, or dedicated detection electronics are fitted at each light emitting diode element or, at each OLEDs connection point, voltage measuring lines have to be led to the driver control electronics, which means an increased wiring outlay. These approaches are expensive and complex.
In order to measure the individual forward voltages, therefore, either a measuring system has to be connected to each OLED, which requires a high wiring outlay and a high number of measuring systems and thus causes high costs, or a single measuring system has to be switched through to the individual OLEDs in each case, for example by means of multiplexing, which however likewise requires a high wiring outlay and outlay for multiplexing and thus causes high costs.
Systems are known, however, in which, in a manner governed by the design, each light emitting diode element is individually contacted with a transistor for switching the light emitting diode element and corresponding control lines to the transistors are present, for example for a dimming and/or a flashing system.
FIG. 2 shows a conventional optoelectronic assembly 10 that largely corresponds to the conventional assembly 10 explained above. The optoelectronic assembly 10 can be for example from the automotive sector, for example a direction indicator of a motor vehicle, for example an animated flashing indicator. The light emitting diode elements 12, 14, 16, 18 are intended to be driven individually with constant current. For cost reasons, the light emitting diode elements 12, 14, 16, 18 are electrically connected in series and only one driver circuit 20 is used, for example a rapidly regulating current source, for example a DC-DC converter. Each light emitting diode element 12, 14, 16, 18 is electrically connected in parallel with respectively a switch, for example a first transistor 32, a second transistor 34, a third transistor 36 and a fourth transistor 36. The current can thus be conducted individually past each light emitting diode element 12, 14, 16, 18 and nevertheless through the other light emitting diode elements 12, 14, 16, 18. For the purpose of dimming, the transistors 32, 34, 36, 38 can also be driven in a pulse-width-modulated manner.
In the case of the conventional optoelectronic assembly shown in FIG. 2, the individual forward voltage can be measured relatively simply compared with FIG. 1. A measuring system can be connected which detects the total voltage, and apart from one switch all the other switches can be closed successively, such that all the light emitting diode elements apart from one are bridged, and then the forward voltage of the individual light emitting diode element can be detected by means of the measuring system. However, here, too, the corresponding light emitting diode element is measured during operation and, as explained above, a drop in the forward voltage is not reliably identifiable depending on the short-circuit resistance.
In many applications, however, in order to reduce costs and wiring outlay, a plurality of OLEDs are connected in series, as shown in FIGS. 1 and 2, and operated with current regulation by a single driver channel. In such applications, the known methods for identifying short circuits are not suitable, do not function sufficiently well or are usable only with increased technical outlay and/or outlay in terms of costs. Consequently, the conventional methods cannot reliably identify one or more short-circuited light emitting diode elements within a series connection or can reliably identify said element(s) only with high technical outlay.